This invention relates generally to turn-off circuits for preventing phosphor burns on the viewing screens of CRTs and particularly to a turn-off circuit used in connection with non-shadow mask projection type CRTs supplied from switched mode power supplies.
A CRT (cathode ray tube) includes an electron gun situated at one end of an evacuated envelope for developing an electron beam that is accelerated toward a light-emitting phosphor target or screen comprising the other end of the CRT. The screen is generally deposited on the inner surface of the CRT faceplate. Magnetic deflection windings, or electrostatic deflection plates, are suitably disposed about the CRT neck and supplied with appropriate horizontal and vertical deflection voltages for deflecting or "sweeping" the electron beam in a pattern over the phosphor screen to define a rectangular-shaped raster. The electron beam is modulated in intensity during its deflection to develop the video display. In direct view color CRTs, a foraminous mask is interposed between the electron gun and the screen for "shadowing" different colored light emitting phosphors from all but their associated electron beams. As is well-known the foraminous mask, or shadow mask as it is commonly referred to, is impacted by and therefore absorbs much of the beam energy. Consequently, phosphor burn due to excessive beam energy in direct view color CRTs is not as serious a problem as it was with monochrome CRTs.
So-called projection television receivers commonly use three independent, different colored CRTs. For example, the color CRTs may comprise individual red, blue and green light emitting types. In such tubes, no shadow mask is required. Rather, three independent images are generated, one in each of the basic colors red, blue and green, and combined, either by direct projection or through a mirror system, to form a resultant color image on a display surface. In these systems, the sources (color CRTs) are small and the final image is large, which requires that the color tubes be driven hard to generate the large light outputs required in projection applications. The combination of large electron beam current and the lack of a shadow mask, makes such projection tubes prone to phosphor burn by the undeflected electron beam in the event appropriate safeguards are not taken when turning the set off. The problem is compounded with the use of switched mode power supplies in which the power supply is on continuously, with the load circuits being switched as needed. The significance of the problem is apparent when considering the high cost of such CRTs and the relative ease with which phosphor burn may occur.
It is conventional practice, when turning off a CRT, to blank the video to cut off the electron beam. In that instance, there is no undeflected electron beam to come to rest at a central spot on the CRT screen and the problem of phosphor burn is not present. However, should the blanking circuitry fail or, as is more likely, should the blanking circuitry operate ineffectively, there is the danger that an electron beam of significant energy may be present when the deflection circuits collapse the raster to a very small area. Hence the problem of phosphor burn is ever present.
Difficulties may also arise should operation of the blanking circuit be compromised, which can occur for a number of reasons. One is that the CRT G2 grid voltage may be very high and cause a shift in the tube cut-off characteristic. It is very common for service personnel to adjust the G2 voltage to its maximum to compensate for a loss in tube brightness due to low cathode emission. Such a tube may experience impaired blanking due to the high G2 voltage and make it prone to phosphor burn upon turn off despite a fully functional and operating blanking circuit. Another problem may be inadvertently introduced by the presence of a safety circuit that is often built into projection TVs to prevent overheating of the CRTs. In some receivers, the CRTs are driven so heavily (to obtain brightness levels suitable for projection viewing) that their frit seals, that is, the glass bonds between the CRT faceplate and funnel may fail because of the differential thermal expansion between the relatively thick faceplate and relatively thin funnel. A high electron beam shut off circuit may be used to turn off the receiver. Turning off the receiver under these conditions may result in compromising the effectiveness of the blanking circuit and pose the threat of phosphor burn. Further, such shutdown circuits may be inadvertently activated by misadjustment of the receiver controls by a serviceman. Also, most projection TV receivers and monitors include a "setup" switch to enable the cut off of the various CRTs to be appropriately established. Inadvertently turning the receiver off while in the setup mode can also seriously compromise the blanking circuit and cause phosphor burn. Accordingly, there is a need in the art for a turn-off circuit for a projection type television CRT that precludes the possibility of phosphor burn.